The present invention is directed, in general, to a method of manufacturing a copolymer and, more specifically, to a method of manufacturing a poly vinylbenzene in the presence of a cross-linking agent and a chain transfer agent.
The polymerization of styrene is a very important industrial process that supplies materials used to create a wide variety of polystyrene-containing articles. This expansive use of polystyrene results from the ability to control the polymerization process. Thus, variations in the polymerization process conditions are of utmost importance since they in turn allow control over the physical properties of the resulting polymer. The resulting physical properties determine the suitability of a polystyrene for a particular use. For a given product, several physical characteristics must be balanced to achieve a suitable polystyrene material. Among the properties that must be controlled and balanced are weight averaged molecular weight (Mw) of the polymer, molecular weight distribution (MWD), melt flow index (MFI), and the storage modulus (Gxe2x80x2).
U.S. Pat. No. 5,540,813 by Sosa, et. al. (Sosa ""813), which is incorporated herein by reference, discloses a process for preparing monovinyl aromatic polymers, such as polystyrene, which utilizes a combination of sequentially ordered multiple reactors, heat exchangers and devolatilizers to strictly control polymer properties such as the molecular weight distribution and melt flow index.
While the Sosa ""813 patent discloses methods for controlling the molecular weight distribution and melt flow index, it does not address the relationship between the molecular weight and the storage modulus. This relationship is of particular importance in polymer foam applications. Such foam applications require high molecular weight polymers having a high storage modulus. It is thought that the storage modulus is related to the degree of branching along the polymer chain. As the degree of branching increases, the likelihood that a branch connects two different polymer chains increases. This inter-chain interaction is known as cross-linking. A polymer product having a higher degree of branching or cross-linking tends to have a higher storage modulus and, therefore, better foam stability characteristics.
Methods for preparing branched polymers are well-known in the art. For example, the preparation of branched polystyrene by free radical polymerization has been reported in U.S. Pat. Nos. 5,473,031 and 5,663,253 issued to Tinetti, et. al., and Pike, et. al., respectively. Both methods increase the branching in the devolatilization step and produce a polymer with an undesirably low molecular weight.
Rather than employing free radical polymerization, U.S. Pat. No. 4,918,159 issued to Idemitsu teaches the use of multi-functional mercaptans to form branched polymers. While materials having an acceptable molecular weight can be prepared by this method, these products are unacceptable for foam applications due to their undesirable flow properties.
The properties of randomly branched polystyrene prepared in the presence of divinylbenzene have been reported by Rubens (L. C. Rubens, J. of Cellular Physics, pp 311-320, 1965). However, polymers having a useful combination of molecular weight and cross-linking are not attainable. At low concentrations of divinylbenzene, low molecular weight polymers having little branching result. However, higher concentrations of the cross-linking agent result in excessive cross-linking and concomitant gel formation. While increasing cross-linking generally correlates with the polymer storage modulus, formation of a gel is highly undesirable in industrial polystyrene processes. Similar results and problems were reported by Ferri and Lomellini (J. Rheol. 43(6), 1999).
Thus what is needed in the art is a process for monovinyl aromatic polymers that produces a branched or very slightly cross-linked product that has a high molecular weight, a high storage modulus at foaming and is suitable for foam applications.
To address the deficiencies of the prior art, the present invention provides a method of producing a copolymer. In an advantageous embodiment, the method comprises placing a vinylbenzene, such as styrene, in a reactor, placing a cross-linking agent, such as divinylbenzene in the reactor, placing a chain transfer agent, such as mercaptan, in the reactor and forming a poly vinylbenzene in the presence of the cross-linking agent and the chain transfer agent. In this embodiment, the concentration of the cross-linking agent ranges from about 150 ppm to about 400 ppm and a concentration of the chain transfer agent ranges from about 100 to about 400 ppm.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.